1. Field of the Invention
The present invention relates to an X-ray tube of the type having a vacuum housing in which a cathode and an anode are arranged, wherein the cathode emits electrons forming a beam and the electrons are accelerated by an electrical field, so as to strike the anode, at a focus (focal spot) and having a magnet system for deflecting and focusing the electron beam formed by several coil elements with current flowing therein, so as to produce a flying focus on the anode from which X-rays emanate.
2. Description of the Prior Art
In computed tomography with a single-line detector, the electron beam of the X-ray tube can be deflected in the .phi.-direction, i.e. in the direction of the perimeter of the outer annular margin of the anode (known as a flying focus), for improving the resolution. This is achieved by a magnet system whose dipole field deflects the beam at high speed. Varying focal spot positions are thus obtained, and the number of projections thus can be increased.
For future application with a multi-line detector and/or surface detector, it would be desirable to simultaneously displace the focal spot in the z direction in order to achieve an enhancement of the resolution in spiral scans.
To achieve these different flying focusings, magnet systems that are spatially separated in the z-direction, which corresponds to the direction parallel to the longitudinal axis of the x-ray tube, can be used, but this increases the length of the tube appreciably, which in turn causes problems in the focusing. Therefore, such a design is extremely undesirable in computed tomography.